Drainage and other trenches of various sizes and shapes are desirable for numerous applications. For example, manufacturing facilities typically require drainage systems which include trenches formed in the building floors to collect, remove and/or recycle excess water or other liquids. In addition, numerous outdoor industrial and commercial sites, such as parking lots, require drainage systems, including trenches, to collect and direct rainwater and other liquids to underground storm sewers to prevent flooding and to decrease runoff.
Furthermore, with increasing emphasis being placed on protecting the environment from ecological hazards, a number of relatively stringent environmental regulations have been adopted which restrict the types of materials which can be discharged into drainage systems. In particular, regulations have been enacted which limit the amount of oil and grease which may be discharged into drainage systems. Consequently, drainage systems, such as the drainage systems installed in gasoline stations, chemical transfer stations, oil storage areas and landfills, can include one or more oil water separators to separate solid debris, free oils and other non-soluble chemicals from the waste water.
In the past, the trenches, including oil water separators, which form these drainage systems have generally been formed by initially placing and securing a form of predetermined shape in a ditch which has previously been formed in the ground. A moldable trench forming composition, such as cement, concrete, or the like, is then poured around the form and is allowed to set. Once the concrete has set, the form is removed from the resulting trench.
One common type of form assembly used to define a trench includes a wooden frame and strut structure. The wooden form includes a wooden frame which is covered with wooden sheets or planks to define a generally rectangular elongate trough. The wooden form is generally enclosed along its side and bottom surfaces, but can have an open top. Typically, a number of supporting wooden ribs are installed within the wooden form to increase the strength of the form so that it can withstand the relatively large pressures exerted by moldable trench forming compositions poured about the form.
During installation, the wooden form is placed and secured within a preformed ditch. Concrete is initially poured in lower portions of the preformed ditch up to the bottom surface of the form. Once this initial pour of concrete has set, additional concrete is then poured between the earthen walls of the ditch and the wooden side surfaces of the form. Once this additional concrete has set, the wooden form is disassembled and removed from the trench.
It is normally desirable to finish the trench with an elongate grate covering its open top in order to prevent people from unwittingly stepping in the open trench, to provide a smooth surface for vehicle travel, and/or to prevent relatively large objects from entering the trench and potentially blocking the flow of liquid therethrough. The grate is generally supported by a pair of spaced apart frame members which are set into and extend from the walls of the concrete trench. In order to stabilize the grate and to prevent the grate from rocking when weight, such as from a passing vehicle, is applied thereto, it is important that the frame members are aligned in a common plane during the pouring and setting of the concrete about the wooden form.
Wooden forms are generally formed of lumber having a relatively rough exterior texture. Correspondingly, the inside surface of the resulting trench formed by the wooden form is relatively uneven, thereby reducing the efficiency with which liquid flows through the trench. In addition, the assembly and disassembly of the wooden forms is both costly and labor intensive. The relatively large costs and labor required for assembly and disassembly of the wooden forms is increased in the formation of long trenches, and even further increased in the formation of trenches having a pitched or slanted bottom surface to facilitate drainage.
As an alternative to wooden forms, precast trench assemblies have been developed. Precast trench assemblies generally include preformed metal and/or plastic assemblies designed to be placed in a ditch. Moldable trench forming composition can thereafter be poured about the precast trench assembly. Once the trench forming composition has set, the precast trench assembly is securely bonded to the trench forming composition which stabilizes and supports the trench. Precast trench assemblies, however, are relatively expensive and cannot generally be reused.
In order to overcome at least some of the shortcomings of wooden forms and precast trench assemblies, a trench forming apparatus and associated method which employ an improved removable form to define the shape of the resulting trench are disclosed in several U.S. Patents to Stegall, including U.S. Pat. No. 5,281,051 which issued Jan. 25, 1994, U.S. Pat. No. 5,348,421 which issued Sep. 20, 1994 and U.S. Pat. No. 5,393,171 which issued Feb. 28, 1995; each of which are assigned to ABT, Inc. of Troutman, N.C., the assignee of the present invention. The trench forming apparatus disclosed in the Stegall patents preferably includes longitudinal frame members having a plurality of anchoring legs extending downwardly therefrom. The elongate form body, typically formed of relatively lightweight expanded polystyrene, preferably includes generally upwardly facing surfaces associated with the opposed side walls of the form body for engaging the frame members. In one advantageous embodiment, the opposed side walls of the form body include aligned longitudinal slots which define respective ones of the upwardly facing surfaces for receiving and engaging the frame members. In particular, horizontal portions of the frame members are engaged by the generally upwardly facing surfaces of the opposed side walls so that the frame members are held in alignment during the trench forming operation. In typical practice, one or more wires are wrapped around the outside of the form body and frame members to hold the frame members in engagement with the generally upwardly facing surfaces defined by the opposed side walls of the form.
Preferably, the assembled form and frame members are placed into a preformed ditch by suspending the assembly from its top. A subslab of trench forming composition is initially poured around lower portions of the anchoring legs and is allowed to set. A second pour of trench forming composition is then poured around the form body and is allowed to set. Finally, the form body is removed from the hardened trench forming composition to expose the resulting trench and the properly aligned frame members. According to one advantageous embodiment, the form includes a pair of slots extending a relatively short distance into the form body from its bottom surface in order to facilitate removal of the form. The trench forming assembly can also include wires disposed within the slots such that, once the trench forming composition has set, the wires can be pulled upwardly through the form. The form body is thereby cut into several pieces that can be more easily removed from the resulting trench.
During the trench forming process described above, the moldable trench forming composition will exert an upwardly directed buoyant force on the buoyant form body. Thus, in addition to holding the frame members in alignment during the trench forming process, the engagement of the generally upwardly facing surfaces of the form body with respective horizontal portions of the frame members at least partially counters this buoyant force exerted on the form body by the moldable trench forming composition. In particular, the frame members and, in turn, the form body are held in position relative to the ground due to the secure engagement of the downwardly extending legs within the concrete subslab. Consequently, the engagement of the generally upwardly facing surfaces of the form body with the frame members is also designed to hold the form body substantially against upward movement during the trench forming process.
The upwardly directed buoyant force exerted on the form body during the trench forming process is directly proportional to the volume of the form about which the moldable trench forming composition is poured. Thus, larger form bodies which create correspondingly larger trenches are subjected to greater buoyant forces. However, the surface area of the generally upwardly facing surfaces of the form body which engage respective horizontal portions of the frame members to align the frame members and to counter the buoyant forces is generally limited to the corresponding surface area of the horizontal portions of the frame members.
Consequently, for a trench forming apparatus having frame members which include horizontal portions of a predetermined size for engaging a relatively large form body, the upwardly directed buoyant forces exerted on the relatively large form body may overcome the compensatory forces provided by the engagement of the upwardly facing surfaces of the form body with the frame members and force the form body upwardly from the trench. If the form body is forced upwardly from the trench by the upwardly directed buoyant forces, the form body is typically destroyed and that portion of the trench must generally be reformed in order to properly shape or form the trench.